1. Field of the Invention
This invention relates to a limited slip differential which is actuated to limit the relative rotational speed between two driven shafts above a predetermined amount and, more particularly, to a drive line clutch having an electromagnet surrounding an annulus filled with magneto-rheological fluid.
2. Description of Related Art
Limited slip differentials are known for providing rotational drive to two output shafts. In particular, such differentials are often used to drive the axles of motor vehicles. A limited slip differential allows relative rotation between two output shafts under certain circumstances. For example, when a vehicle turns, the output shaft of the outer wheel typically turns at a higher rotational speed than the output shaft of the inner wheel. A limited slip differential allows this relative rotation. If one of the wheels begins to slip, however, the relative rotational speed between the output shafts of the two wheels exceeds a predetermined limit. A limited slip differential resists such relative rotational speed.
One known limited slip differential employs a viscous coupling which xe2x80x9cmonitorsxe2x80x9d any relative rotation between the two output shafts. When the relative rotational speed exceeds a certain limit, viscous fluid in the viscous coupling expands to create an axial force against an actuation member. In turn, the actuation member is forced against, and engages a friction clutch, directly connecting the first and second output shafts.
There are drawbacks to such prior art differentials. Relying on fluid expansion does not provide fine control over the amount of relative rotation required before clutch actuation. Further, the reliance on the expanding fluid also may not be easily controlled to allow variation of the desired limit on relative rotational speed.
Electromagnetic clutches have been developed to improve power delivery to all wheels. With an increasingly sophisticated array of vehicle condition sensors and a concomitant increase in the sophistication of computer software, it is apparent that improvements are possible in the structure and operation of electromagnetic clutches. However, conventional electromagnetic clutches suffer from poor operation and packaging problems.
The need exists for an effective and efficiently packaged electromagnetic clutch for a limited slip differential assembly.
The present invention is directed to an electromagnetic clutch assembly which provides enhanced speed of operation and improved modulating control.
The invention provides a multiple disc clutch pack having a magneto-rheological operator and a ball ramp operator.
In a disclosed embodiment of the present invention, a limited slip differential incorporates a friction clutch which is selectively actuated to directly contact two driven shafts. A viscous coupling is connected to the friction coupling by a pair of rotatable discs with inclined cam surfaces formed between the discs. Should there be an undesirable amount of relative rotational speed, the electromagnetic coupling creates a rotational shear force causing a first disc to rotate relative to the second disc. The relative rotation of the discs causes the inclined cam surfaces to move relative to each other, in turn resulting in axial movement of the second disc to engage the friction clutch.
With the inventive differential, fluid expansion is replaced by the more predictable and controllable electromagnetic coupling. Moreover, the parameters of the viscous coupling and discs may be easily varied to control the amount of relative rotational speed which is allowed prior to actuation of the friction clutch. As an example, the cam surfaces can be varied to control the desired amount of relative rotational speed.
Magneto-rheological (MR) fluids exhibit varying shear strength in response to an applied magnetic field, and the invention utilizes this property to vary the compressive force on a clutch pack contained in the differential, thereby varying the torque-biasing characteristic. The orientation and packaging of the invention minimizes the amount of MR fluid, maximizes the space available for the clutch pack, and may allow the use of a one-piece differential case casting as opposed to more costly two-piece designs.
The invention includes an electromagnet surrounding an annulus filled with MR fluid. The electromagnet is fixed to the housing, and the MR fluid is in contact with a ball-ramp mechanism that converts the shear stress of the MR fluid to a linear force that is applied to the clutch pack. Varying the electrical power to the electromagnet causes the shear strength of the MR fluid to change proportionally, thus varying the torque applied to the ball ramp mechanism and the compressive force applied to the clutch pack. With the invention, the MR fluid may be disposed external to the differential housing or contained within the differential housing.
These and other features of the present invention can be best understood from the following specification and drawings, of which the following is a brief description.